1. Field of the Invention
This invention relates to signal processing of time domain electronic signals, such as video information signals. In particular, the invention relates to improvements in scrambling and descrambling such signals to prevent unauthorized use thereof, including several improvements in security and concealment.
2. Description of Related Art
Commonly assigned U.S. Pat. No. 5,058,157 issued Oct. 15, 1991 to John O. Ryan (incorporated herein by reference) discloses a method and apparatus for encrypting (scrambling) and decrypting (descrambling) information signals normally arranged as a succession of lines of active information, with each line having a line timing reference, such as color video (television) signals. The active video portion of each line is time shifted with respect to the horizontal sync portion of that line using a predetermined slowly varying time-shifting function. The time-shifting information is conveyed to the decryption site by encoding the instantaneous value of the time-shifting waveform for the beginning of each field in the vertical blanking portion of that field. To provide a reasonable maximum time-shifting range, portions of the trailing edge of the active video in the preceding line and portions of the leading edge of the active video in the current line are discarded. During decryption, the original line timing and colorburst signals are discarded and new signals are generated which are time displaced from the active video portion by the original amount before encryption. This provides a secure video type information encryption and decryption technique compatible with all video tape formats and transmission systems, and which is free of picture impairments caused by the interaction of the scrambling algorithm and the chrominance consecutive line averaging systems used in color-heterodyne recording.
The type of time shifting performed may comprise any one of a number of slowly varying functions, such as a sinusoidal waveform or a linearly changing ramp signal. The rate of change in the signal, i.e. the "wobble", is relatively slow when compared to the line rate of the input signals to be processed. For video type signals, a sinusoidal waveform having a frequency of no more than about 20 Hz is used. The absolute amount of time shifting performed is preferably limited to a maximum value which, in the case of NTSC video signals, does not exceed a total of 4 microseconds (plus or minus 2 microseconds in each direction).
The instantaneous value of the time shifting waveform function at the beginning of each field is conveyed along with the field information, typically during the vertical blanking interval. For example, with respect to a sinusoidal time shifting function, the starting amplitude of the waveform during a given field is transmitted during the vertical blanking interval as a single byte of information which, when combined with a separately provided authorization key, enables a descrambling circuit to synthesize the scrambling waveform function. Decryption is performed by restoring the original timing relationship between the horizontal sync (and colorburst) and the active video portion of the corresponding line. This is done by generating new line timing reference signals (horizontal sync and colorburst) which bear the same timing relationship to the active video portion as do the original line timing reference signals before encryption. The resulting descrambled signals still contain time base errors, but these errors are within the capture or correction range of the follow-on television monitor/receiver.
FIGS. 1A and 1B herein correspond to FIGS. 3A and 3B of the above cited U.S. Pat. No. 5,058,157, and illustrate the manner in which the scrambled signals are descrambled at the reception site, i.e. the descrambler. With reference to FIG. 1A, three successive lines of NTSC video are shown which have been time shifted successively by increasing amounts. (The active video portions of each of the lines in FIGS. 1A and 1B are only fractionally illustrated.) The topmost line represents a line N having no time shifting between the active video portion and the end of horizontal blanking, and the time between the beginning of a horizontal sync portion and the active portion is designated as t.sub.1. The next line N+1 has undergone time shifting in the delay direction so that the time between the beginning of the horizontal sync portion and the beginning of active video portion is t.sub.2, greater than t.sub.1. Line N+2 has undergone even more time shifting in the delay direction by an amount labeled t.sub.3 which is greater than t.sub.2. These three successive lines represent lines from the upper portion of a raster image. The line timing reference part of each of lines N, N+1 and N+2 are all temporally aligned in FIG. 1A; the leading edge of the horizontal sync portion of each line is exactly aligned with the leading edge of the horizontal sync portion of the other lines. The same is true of the location of the colorburst portions (hatched areas). The active video portions, however, are deliberately misaligned in lines N+1 and N+2 with respect to line N.
FIG. 1B illustrates the signals for the same three lines after descrambling, i.e. decryption. As can be seen in this figure, the leading edges of the horizontal sync portions of the three lines are no longer precisely aligned, but are rather staggered; however, the distance between the leading edge of the horizontal sync portion and the beginning of active video is the same for all three lines, i.e. the value t.sub.1. Similarly, the colorburst portions (hatched areas) of the three lines are no longer temporally aligned, but are rather staggered in the same fashion as the horizontal sync portions. Relative positioning of the active video portion of the three lines remains the same.
Although the descrambled signals are still relatively misaligned, the precise timing relationship t.sub.1 between the leading edge of horizontal sync and the beginning of active video ensures that each line of information, as processed by the follow-on television receiver or monitor, can be properly displayed, provided that the timing error in a given line does not exceed the capture range of the television receiver or monitor synchronization circuitry. The time shifting applied to the original signals during encryption is relatively slowly varying (20 Hz for NTSC TV) compared to the video line rate.
FIGS. 2A and 2B herein are the same as FIGS. 4A and 4B of U.S. Pat. No. 5,058,157. These figures show in block diagram form a scrambler system capable of providing the above-described scrambling. As seen in FIGS. 2A and 2B, input video to be scrambled is coupled to an input terminal 11 of a video input processor unit 12. Processor 12 functions to normalize the incoming video signal relative to gain, DC offset and bandwidth and provides a stable low impedance buffer unit for the video appearing on output terminal 13. In addition, the incoming vertical and horizontal sync portions are separated from the input video by processor unit 12 and supplied as input to a sync/timing generator and phase locked loop 15.
The signals from processor unit 12 appearing on output terminal 13 are coupled to a conventional NTSC decoder and anti-alias filter 16 in which the luminance component Y and chrominance quadrature components I,Q are separated for three channel parallel processing in the digital domain. The Y output of unit 16 is coupled to an analog-to-digital converter 18 in which the luminance is converted from analog to digital form at a preselected clock rate by means of an input sample clock signal supplied on clock input line 19. The output of converter 18 is coupled to an input portion of a dual-ported luminance memory unit 20. This is then the Y channel memory connected to the Y channel D/A converter 22. Memory unit 20 is configured as a memory in which a word is written from A/D converter 18 into every memory cycle and a word is read from memory unit 20 to a digital-to-analog converter unit 22 every memory cycle.
Read/write control signals and multi-bit address signals are supplied to the luminance memory unit 20 from a memory controller unit 24. The output of luminance channel memory unit 20 is coupled to the input of a digital-to-analog converter 22, in which the multi-bit digital words output from memory 20 are converted into analog samples at the clock rate by clock signals supplied from unit 15 on clock input line 23. The output of converter unit 22 is coupled to the input of an NTSC encoder and low pass filter unit 25 in which the luminance signal is combined with the I and Q chrominance components and renormalized with respect to band-width and DC offset. The I, Q chrominance quadrature components are processed in an essentially identical manner to that described above for the luminance component Y in respectively units 18', 20' and 22' and 18", 20", and 22", which function in the same manner respectively as do units 18, 20 and 22.
Sync timing unit 15 generates the input clock signals used to provide the sample clock for A/D converter unit 18, the read and write clock signals from memory unit 20, and the clock signals for D/A converter unit 22. Preferably, unit 15 is comprised of a discrete phase detector, a number of sampling gates, and error amplifier and a crystal clock oscillator.
The above described units are coupled to a user interface device 32, such as a keyboard terminal, via controller unit 34 and a plurality of control registers 36.
The above described device and the associated scrambling method have several shortcomings.
First, the device is relatively expensive and complicated in that there are three sets of A/D converters and associated memories, one for each of the Y, I, and Q components. Thus, there are three independent channels for digital processing, each channel requiring relatively expensive components, thus increasing the cost and complexity of the scrambling device.
Secondly, the method of scrambling as depicted in FIGS. 1A and 1B while reasonably secure has the potential defect that in the process of moving the active portion of the video to the right as shown in the drawings, the leading and trailing edges of the horizontal sync signal have both been moved to the right also. This displacement of the normally well known position of horizontal sync within the horizontal blanking interval could be detected by a clever pirate, i.e. unauthorized user, to determine the amount of wobble (time displacement) in each line. The pirate would be able at least in theory to descramble the signal to determine what the amount of wobble and reverse the process, thus obtaining a descrambled and viewable signal. Thus, the method as depicted in FIGS. 1A and 1B is lacking in the very high degree of security desirable for a commercial scrambling system.
Another shortcoming of the above-described scrambling system is that while providing security, i.e. generally preventing unauthorized use, the scrambled signal when viewed on a normal television set is not completely concealed. That is, a determined viewer who is willing to watch a television picture which is in effect horizontally jumping back and forth can still watch the program and understand at least partly what is going on. This is undesirable for transmission for instance of adult type material where it is desired to prevent children from watching even the scrambled picture. This is especially problematic because it has been determined by experimentation that such adult type material, i.e. depictions of sexual activity, is particularly easy to follow on the picture by a viewer even though the picture is scrambled. This is another way of saying that the scrambling while relatively secure does not provide an adequate level of concealment for all program material.
Another problem associated with the above described device is one common to comb-type NTSC decoders in which the composite video is subjected to a one-line delay. Simple addition of the delayed video to the same video before the delay causes the chrominance portion of the two signals to cancel, leaving only luminance. Similarly and simultaneously, subtraction of the delayed signal from the undelayed signal (or vice versa) causes the luminance portion to cancel, leaving only chrominance. This problem is not specific to a scrambling device but is typically encountered in video processors which perform NTSC decoding and is the reduction of vertical detail, resulting in smeared vertical edges in the picture. This is due to the two-line summation process of the Y, I and Q components in which fine grain picture detail tends to be lost when the composite video is converted to digital and then in the digital domain a luminance/chrominance separation is performed. It is known that this problem can be overcome by complicated and expensive circuitry which takes the incoming composite video signal in the analog domain, using a band pass or high pass filter to isolate the chrominance component before separation. The band pass filtered signal is then delayed and subjected to the subtraction process. The band pass filtering removes the vertical luminance edges because they are low frequency in nature. Thus the chrominance separation is performed only on the high frequencies and having done this, the resultant separated chrominance has no luminance component. Finally the luminance signal is isolated by subtracting the finished, high frequency chrominance signal from the incoming composite video so there is no loss of vertical detail. This process is effective but when done digitally requires two A/D conversions: one for the band passed (or high passed) chrominance and one for the broad band composite video. It would be desirable to eliminate or simplify this process in order to reduce the number of components needed and reduce the amount of processing on the signal.
Thus, the method and apparatus disclosed in the above cited patent while adequate is still subject to significant improvement in both security, concealment, and complexity.
It is to be understood that the above cited patent is commonly assigned with the present invention and that the above description is not an admission that the subject matter disclosed and claimed in the above cited patent is necessarily prior art with respect to the subject matter of the present disclosure and claims.